Honeycomb structure producing method, and honeycomb structure

ABSTRACT

In a method for producing a honeycomb structure  20  which comprises disposing a material for forming outer wall  11  on the outer peripheral surface  3  of a cell structure  1  having a plurality of cells serving as fluid flowing channels to produce a cell structure being provided with a material for forming outer wall  10  and firing the resulting cell structure being provided with a material for forming outer wall  10 , wherein a cell structure being provided with a material for forming outer wall  10  in which the absolute value of the difference between the volumetric shrinkage percentage (firing shrinkage percentage) before and after firing of the cell structure  1  and the firing shrinkage percentage of the material for forming outer wall  11  is not more than 0.5% is produced and fired. There is provided a method for producing a honeycomb structure which comprises providing an outer wall on the outer peripheral surface and firing it, according to which the outer wall hardly cracks during firing and high-strength and large-sized honeycomb structure can be produced, and further provided is a high-strength and large-sized honeycomb structure in which the outer wall hardly cracks during firing.

TECHNICAL FIELD

The present invention relates to a method for producing a high-strengthand large-sized honeycomb structure, and a high-strength and large-sizedhoneycomb structure.

BACKGROUND ART

Honeycomb structures comprising ceramics are used for collecting dustsand other particulate materials contained in exhaust gases dischargedfrom automobiles and exhaust gases generated upon incineration of wasteproducts, for purification of the exhaust gases by removing NO_(x), CO,HC, etc. using supported catalysts or for purification ofhigh-temperature exhaust gases discharged from production steps invarious manufacturing and mining industries. The honeycomb structuresare structures formed for collecting and removing particulate materialscontained in dust-containing fluids. They have many fluid flowing cellswhich are partitioned with partition walls and which pierce through thehoneycomb structures in axial direction, and the partition walls of thefluid flowing cells have filtering ability, and one end part of acertain fluid flowing cells is sealed and another end part of otherfluid flowing cells is sealed. Furthermore, NO_(x), CO, HC, etc. inexhaust gases can be removed by supporting catalysts on the surfaceand/or inside of the partition walls of the fluid flowing cells.

Recently, honeycomb structures are used suitably for purification ofexhaust gases of diesel engine cars. For purification of exhaust gasesdischarged from diesel engines, it is necessary to adsorb and removeNO_(x), CO, HC, etc., and simultaneously to collect particulatesdischarged from engines, and hence honeycomb structures are used asfilters for removal of particulates having catalyst supported therein(DPF: diesel particulate filters). Moreover, honeycomb structures, ofwhich end face is not sealed, are also used as catalyst carriers indiesel engine cars. The diesel engines are mounted on large-sizedvehicles such as trucks and buses which are great in cylinder volume andhence the honeycomb structures must also be large-sized.

However, if honeycomb structures are large-sized, mechanical strengththereof decreases and therefore distortions and damages are apt to occurduring use, which necessitates employment of a reinforcing means. As thereinforcing means, there is a method of disposing an outer wall on theouter peripheral surface of the honeycomb structures, but this methodcannot be applied to large-sized honeycomb structures. For example,there is disclosed a method of providing an outer peripheral wall on thehoneycomb structures, followed by firing, but this method gives noconsideration to the difference between the proportion of dimensionalshrinkage of the outer peripheral wall before and after firing (firingshrinkage percentage) and the proportion of volumetric shrinkage of theinner cell structure (ceramic honeycomb body), and, hence, when the cellstructure is made larger in size, distortion is apt to occur duringfiring due to the difference in proportion of dimensional shrinkagebefore and after firing, resulting in breakage of the structure (e.g.,UM (Utility Model)-B-7-183).

Furthermore, there is disclosed a method of providing an outerperipheral wall on a honeycomb structure to improve mechanical strength(e.g., Japanese Patent No.2604876). However, according to this method,the outer wall is low in strength, and may crack during handling or atthe step of supporting of catalysts, which may lead to breaking.Moreover, if the amount of the bonding material such as colloidal silicais increased in order to enhance the strength, the coefficient ofthermal expansion of the outer peripheral coat becomes higher than thatof the inner cell structure (cordierite honeycomb), and there is thepossibility of falling off of the outer peripheral coat part whensubjected to thermal cycling. Further, there is disclosed a ceramichoneycomb structure, the outer periphery of which is coated withcordierite particles and/or ceramic fibers, and colloidal silica orcolloidal alumina present between the outer periphery and the coat ofcordierite particles and/or ceramic fibers (e.g., Japanese PatentNo.2613729). However, this ceramic honeycomb structure is low instrength of the outer wall, and the outer wall may crack during handlingor at the step of supporting of catalysts, resulting in breakage of thehoneycomb structure. Moreover, if the amount of the bonding materialsuch as colloidal silica is increased in order to enhance the strength,the coefficient of thermal expansion of the outer peripheral coatbecomes higher than that of the inner cell structure (cordieritehoneycomb), and there is the possibility of falling off of the outerperipheral coat part when subjected to thermal cycling.

The present invention has been made in an attempt to solve the aboveproblems, and the object of the present invention is to provide a methodfor producing a honeycomb structure provided with an outer wall bydisposing a material for forming outer wall on the outer peripheralsurface, according to which the outer wall hardly cracks during firingand a high-strength and large-sized honeycomb structure can be produced,and, further object is to provide a high-strength and large-sizedhoneycomb structure with substantially no cracks in the outer wall.

DISCLOSURE OF INVENTION

In order to attain the above object, the following method for producinga honeycomb structure and the following honeycomb structure are providedby the present invention.

[1] A method for producing a honeycomb structure which comprisesdisposing a material for forming outer wall forming an outer wall uponfiring on the outer peripheral surface of a cell structure having aplurality of cells serving as fluid flowing channels and partitioned bypartition walls to produce a cell structure being provided with amaterial for forming outer wall and firing the resulting cell structurebeing provided with a material for forming outer wall to obtain ahoneycomb structure comprising the cell structure provided with theouter wall on the outer peripheral surface thereof, characterized inthat the cell structure being provided with a material for forming outerwall is produced by using the cell structure and the material forforming outer wall which are combined so that the absolute value of thedifference between the proportion of the size of the cell structureshrinkage after firing to the size of the cell structure before firingand the proportion of the shrinkage in the size of the material forforming outer wall after firing to the size of the material for formingouter wall before firing is not more than 0.5%, and the resulting cellstructure being provided with a material for forming outer wall isfired.

[2] A method for producing a honeycomb structure described in the above[1], wherein the main component of the cell structure and/or outer wallis a ceramics.

[3] A method for producing a honeycomb structure described in the above[1] or [2], wherein the cell structure being provided with a materialfor forming outer wall is produced so that the absolute value of thedifference between the coefficient of thermal expansion of the cellstructure after firing and that of the outer wall after firing is notmore than 0.7×10⁻⁶/° C.

[4] A method for producing a honeycomb structure described in any one ofthe above [1]-[3], wherein the cell structure being provided with amaterial for forming outer wall is produced so that the main componentof the cell structure after firing is cordierite.

[5] A method for producing a honeycomb structure described in any one ofthe above [1]-[4], wherein the cell structure is unfired, and the cellstructure being provided with a material for forming outer wall isproduced by disposing the material for forming outer wall on the outerperipheral surface of the unfired cell structure and the resulting cellstructure being provided with a material for forming outer wall isfired.

[6] A method for producing a honeycomb structure described in any one ofthe above [1]-[4], wherein the cell structure is previously fired, andthe cell structure being provided with a material for forming outer wallis produced by disposing the material for forming outer wall on theouter peripheral surface of the fired cell structure and the resultingcell structure being provided with a material for forming outer wall isfired.

[7] A method for producing a honeycomb structure described in any one ofthe above [1]-[6], wherein the material for forming outer wall whichforms the outer wall mainly composed of cordierite by firing isdisposed.

[8] A method for producing a honeycomb structure described in the above[7], wherein the material for forming outer wall which is prepared fromat least one material selected from the group consisting of talc,calcined talc, kaolin, calcined kaolin, alumina, aluminum hydroxide,mullite and silica so that the material contains cordierite as a maincomponent after firing is disposed on the outer peripheral surface ofthe cell structure to produce the cell structure being provided with amaterial for forming outer wall and this cell structure being providedwith a material for forming outer wall is fired to obtain the honeycombstructure having the outer wall containing cordierite as a maincomponent.

[9] A method for producing a honeycomb structure described in any one ofthe above [6]-[8], wherein the material for forming outer wall containsa quartz powder in an amount of 1-15% by mass based on the wholematerial for forming outer wall.

[10] A method for producing a honeycomb structure described in any oneof the above [1]-[9], wherein the cell structure being provided with amaterial for forming outer wall is produced by using the cell structureand the material for forming outer wall which are combined so that theabsolute value of the difference between the proportion of shrinkage inthe size of the cell structure after firing to the size of the cellstructure before firing and the proportion of shrinkage in the size ofthe material for forming outer wall after firing to the size of thematerial for forming outer wall before firing is not more than 0.3%, andthe resulting cell structure being provided with a material for formingouter wall is fired.

[11] A method for producing a honeycomb structure described in any oneof the above [1]-[10], wherein the cell structure being provided with amaterial for forming outer wall is produced so that the absolute valueof the difference between the coefficient of thermal expansion of thecell structure after firing and that of the outer wall after firing isnot more than 0.4×10⁻⁶/° C.

[12] A method for producing a honeycomb structure described in any oneof the above [1]-[11], wherein the maximum diameter of a section formedby cutting the cell structure after firing along a plane perpendicularto the central axis is not less than 150 mm.

[13] A method for producing a honeycomb structure described in any oneof the above [1]-[12], wherein the outer peripheral surface of the cellstructure is formed of the surface of the outer peripheral wall providedon the outside of the cells positioned at the outermost periphery amonga plurality of the cells.

[14] A method for producing a honeycomb structure described in any oneof the above [1]-[12], wherein the outer peripheral surface of the cellstructure is formed of the surface of the partition walls of the cellspositioned at the outermost periphery among a plurality of the cells.

[15] A method for producing a honeycomb structure described in any oneof the above [1]-[12], wherein the outer peripheral surface of the cellstructure is formed by grinding at least a part of the outer peripheralwall of the cell structure which comprises a plurality of cells servingas fluid flowing channels and is provided with the outer peripheral wallon the outside of the cells positioned at the outermost periphery amonga plurality of the cells.

[16] A method for producing a honeycomb structure described in any oneof the above [1]-[15], wherein the surface of the outer wall of thehoneycomb structure is subjected to a surface working.

[17] A method for producing a honeycomb structure described in any oneof the above [1]-[16], wherein the honeycomb structure is cylindricaland the difference between the maximum diameter and the minimum diameterof the cylindrical honeycomb structure is not more than 1 mm.

[18] A method for producing a honeycomb structure described in any oneof the above [1]-[17], wherein the raw material for the cell structureand the material for forming outer wall are kaolin and the averageparticle diameter of kaolin used as the material for forming outer wallis not less than 1/10 and not more than ½ of the average particlediameter of kaolin used as the raw material for the cell structure.

[19] A honeycomb structure provided with an outer wall on the outerperipheral surface of a cell structure which is produced by firing acell structure being provided with a material for forming outer wallcomprising a cell structure having a plurality of cells serving as fluidflowing channels and partitioned by partition walls and a material forforming outer wall disposed on the outer peripheral surface of the cellstructure and forming an outer wall upon firing, characterized in thatthe absolute value of the difference between the proportion of shrinkagein the size of the cell structure after firing to the size of the cellstructure before firing and the proportion of shrinkage in the size ofthe material for forming outer wall after firing to the size of thematerial for forming outer wall before firing is not more than 0.5%.

[20] A honeycomb structure described in the above [19], wherein the maincomponent of the cell structure and/or outer wall is a ceramics.

[21] A honeycomb structure described in the above [19] or [20], whereinthe absolute value of the difference between the coefficient of thermalexpansion of the cell structure after firing and that of the outer wallafter firing is not more than 0.7×10⁻⁶/° C.

[22] A honeycomb structure described in any one of the above [19]-[21],wherein the main component of the cell structure after firing iscordierite.

[23] A honeycomb structure described in any one of the above [19]-[22],wherein the cell structure is unfired, and the cell structure beingprovided with a material for forming outer wall comprising the unfiredcell structure and the material for forming outer wall disposed on theouter peripheral surface of the unfired cell structure is fired toproduce the honeycomb structure.

[24] A honeycomb structure described in any one of the above [19]-[22],wherein the cell structure being provided with a material for formingouter wall comprising the previously fired cell structure and thematerial for forming outer wall disposed on the outer peripheral surfaceof the fired cell structure is fired to produce the honeycomb structure.

[25] A honeycomb structure described in any one of the above [19]-[24],wherein the main component of the outer wall after firing is cordierite.

[26] A honeycomb structure described in the above [25], wherein the cellstructure being provided with a material for forming outer wallcomprising the cell structure and the material for forming outer wallwhich is prepared using at least one material selected from the groupconsisting of talc, calcined talc, kaolin, calcined kaolin, alumina,aluminum hydroxide, mullite and silica so that the material containscordierite as a main component after firing and which is disposed on theouter peripheral surface of the cell structure is fired, whereby themain component of the outer wall becomes cordierite.

[27] A honeycomb structure described in any one of the above [24]-[26],wherein the material for forming outer wall contains a quartz powder inan amount of 1-15% by mass based on the whole material for forming outerwall.

[28] A honeycomb structure described in any one of the above [19]-[27],wherein the cell structure being provided with a material for formingouter wall is produced so that the absolute value of the differencebetween the proportion of shrinkage in the size of the cell structureafter firing to the size of the cell structure before firing and theproportion of shrinkage in the size of the material for forming outerwall after firing to the size of the material for forming outer wallbefore firing is not more than 0.3%.

[29] A honeycomb structure described in any one of the above [19]-[28],wherein the absolute value of the difference between the coefficient ofthermal expansion of the cell structure after firing and that of theouter wall after firing is not more than 0.4×10⁻⁶/° C.

[30] A honeycomb structure described in any one of the above [19]-[29],wherein the maximum diameter of the cell structure in a section formedby cutting the cell structure after firing along a plane perpendicularto the central axis is not less than 150 mm.

[31] A honeycomb structure described in any one of the above [19]-[30],wherein the outer peripheral surface of the cell structure is formed ofthe surface of the outer peripheral wall provided on the outside of thecells positioned at the outermost periphery among a plurality of thecells.

[32] A honeycomb structure described in any one of the above [19]-[30],wherein the outer peripheral surface of the cell structure is formed ofthe surface of the partition walls of the cells positioned at theoutermost periphery among a plurality of the cells.

[33] A honeycomb structure described in any one of the above [19]-[30],wherein the outer peripheral surface of the cell structure is formed bygrinding at least a part of the outer peripheral wall of the cellstructure comprising a plurality of cells serving as fluid flowingchannels and provided with the outer peripheral wall on the outside ofthe cells positioned at the outermost periphery among a plurality of thecells.

[34] A honeycomb structure described in any one of the above [19]-[33],wherein the surface of the outer wall of the honeycomb structureprovided with the outer wall on the outer peripheral surface of the cellstructure is subjected to a surface working.

[35] A honeycomb structure described in any one of the above [19]-[34],wherein the honeycomb structure is cylindrical and the differencebetween the maximum diameter and the minimum diameter of the cylindricalhoneycomb structure is not more than 1 mm.

[36] A honeycomb structure described in any one of the above [19]-[35],wherein the cell structure and the outer wall contain kaolin and theaverage particle diameter of the kaolin contained in the outer wall isnot less than 1/10 and not more than ½ of the average particle diameterof the kaolin contained in the cell structure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view of a cell structure made at an intermediatestage which is cut along a plane perpendicular to the central axis inone embodiment of the method for producing the honeycomb structure ofthe present invention.

FIG. 2 is a sectional view of the honeycomb structure cut along a planeperpendicular to the central axis of the present invention.

FIG. 3 is a sectional view of a cell structure made at an intermediatestage which is cut along a plane perpendicular to the central axis inanother embodiment of the method for producing the honeycomb structureof the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

According to the method for producing a honeycomb structure and thehoneycomb structure of the present invention, a cell structure, on theouter peripheral surface of which is disposed a material for formingouter wall, is fired to produce a honeycomb structure in which thedifference between the proportion of shrinkage in size before and afterfiring of the cell structure and the proportion of shrinkage in sizebefore and after firing of the material for forming outer wall is set atnot more than 0.5%. Therefore, the outer wall hardly cracks duringfiring and thus a high-strength and large-sized honeycomb structure canbe produced, and a high-strength and large-sized honeycomb structure inwhich the outer wall hardly cracks can be provided.

The embodiments of the present invention will be specifically explainedreferring to the drawings, but they should not be construed as limitingthe invention in any manner, and it should be understood that optionalchanges, modifications and improvements may be made based on the normalknowledge of one skilled in the art without departing from the spiritand scope of the invention.

According to one embodiment of the method for producing a cell structureof the present invention, first a cylindrical cell structure 1 isproduced by a general extrusion molding method and dried. FIG. 1 is asectional view of a cell structure 1 made at an intermediate stage whichis cut along a plane perpendicular to the central axis, in the methodfor producing the honeycomb structure of the present invention. The cellstructure 1 comprises many cells 2 which are partitioned (compartmented)by partition walls 4 and pierce through the cell structure in thecentral axis direction and has an outer peripheral wall 5 formed on theoutside of the cells positioned at the outermost periphery. The surfaceof the outer peripheral wall 5 forms the outer peripheral surface 3 ofthe cell structure 1.

Next, a material for forming outer wall (a slurry for forming the outerwall) which forms the outer wall by firing is coated on the outerperipheral surface 3 of the resulting cell structure 1 to dispose thematerial for forming outer wall 11 to produce a cell structure beingprovided with a material for forming outer wall 10, which is fired toobtain a honeycomb structure 20 comprising the cell structure 1 providedwith an outer wall 21 on the outer peripheral surface.

The firing conditions are such that the cell structure being providedwith a material for forming outer wall is kept at 1410-1430° C. for 3-7hours.

In the method for producing the honeycomb structure of this embodiment,as the cell structure 1 and the material for forming outer wall 11, theyare used in combination so that the absolute value of the differencebetween the proportion of shrinkage in the size of the cell structure 1after firing to the size of the cell structure 1 before firing(hereinafter referred to as “firing shrinkage percentage of cellstructure 1”) and the proportion of the shrinkage in the size of thematerial for forming outer wall 11 after firing to the size of thematerial for forming outer wall 11 before firing (hereinafter referredto as “firing shrinkage percentage of material for forming outer wall11”) is not more than 0.5%, preferably not more than 0.3%. If thedifference is more than 0.5%, since the absolute value of the differencebetween the firing shrinkage percentage of cell structure 1 and firingshrinkage percentage of material for forming outer wall 11 is great, theouter wall 21 cracks during firing (after firing), which is notpreferred. Here, the firing shrinkage percentage of the cell structure 1is a value obtained by subtracting the size of the cell structure 1after firing from the size of the cell structure 1 before firing anddividing the resulting subtraction value by the size of the cellstructure 1 before firing, and can be shown by the following formula(1).(Firing shrinkage percentage of cell structure 1) (%)=100×((size of cellstructure 1 before firing)−(size of cell structure 1 afterfiring))/(size of cell structure 1 before firing)   (1)

The size of the cell structure 1 before and after firing can be obtainedby measuring the diameter of circle in the case of cylindrical cellstructure. The firing shrinkage percentage of the material for formingouter wall 11 is measured by firing only the material for forming outerwall 11 without disposing the material for forming outer wall 11 on theouter peripheral surface of the cell structure 1. For example, a plateof about 50 mm×50 mm×50 mm is made from the material for forming outerwall, and the size thereof before and after firing is measured. Analysisof the firing shrinkage percentage of the material for forming outerwall 11 is conducted by the above formula (1) where “cell structure 1”is replaced with “material for forming outer wall 11”.

In the honeycomb structure 20 produced by the method for producing ahoneycomb structure of this embodiment, the absolute value of thedifference between the coefficient of thermal expansion of the cellstructure 1 after firing and that of the outer wall 21 after firing ispreferably not more than 0.7×10⁻⁶/° C., more preferably not more than0.4×10⁻⁶/° C. If it is more than 0.7×10⁻⁶/° C., thermal shock resistanceof the honeycomb structure deteriorates and cracks are sometimesproduced by thermal cycles.

Measurement of the coefficient of thermal expansion is carried out bymeasuring dimensional change

L at a temperature from 40° C. to 800° C. The coefficient of thermalexpansion is a value obtained by dividing the dimensional change

L by the sample size L at 40° C. and the temperature (800° C.-40° C.),and is shown by the following formula (2).Coefficient of thermal expansion (/° C.)=

L/(L×760)   (2)

The cell structure 1 and outer wall 21 after firing are mainly composedof preferably a ceramics, more preferably cordierite. Particularly, asfor the outer wall 21, it is preferred that a composition comprising atleast one selected from the group consisting of talc, calcined talc,kaolin, calcined kaolin, alumina, aluminum hydroxide, mullite and silicais prepared so that it contains cordierite as a main component afterfiring, the resulting composition is contained in the material forforming outer wall 11, then this material for forming outer wall 11 iscoated (disposed) on the outer peripheral surface 3 of the cellstructure 1 to produce the cell structure being provided with a materialfor forming outer wall 10, and this cell structure being provided with amaterial for forming outer wall 10 is fired to allow the outer wall 21after firing to contain cordierite as a main component. Furthermore,when the cell structure is previously fired and the material for formingouter wall is disposed on the outer peripheral surface of the fired cellstructure, it is preferred to use (contain) a quartz powder in thematerial for forming outer wall in an amount of 1-15% by mass based onthe whole material for forming outer wall because the absolute value ofthe difference in firing shrinkage percentage between the cell structureand the material for forming outer wall can be adjusted to not more than0.5%.

All of the cell structure 1, the cell structure being provided with amaterial for forming outer wall 10 and the honeycomb structure 20 arepreferably cylindrical with having a central axis in the same directionas the central axis of the cells 2, and the sectional shapes thereofwhen they are cut along a plane perpendicular to the central axis ofthem are not particularly limited, and may be circle, oval (ellipse),polygons such as square and rectangle, and other indeterminate shapes.

The thickness of outer wall 21 is preferably 0.5-2.0 mm. If thethickness is less than 0.5 mm, the outer wall 21 is thin and it becomessometimes difficult to maintain the strength of the honeycomb structure.If it is more than 2.0 mm, a temperature gradient in the outer wall 21is apt to occur to cause reduction in thermal shock resistance.

The honeycomb structure 20 produced by the method of this embodiment issuitable when the cell structure 1 after firing has a maximum diameterof 150 mm or more of the section formed by cutting the cell structurealong a plane perpendicular to the central axis. Since the material forforming outer wall 11 used in this embodiment is disposed on the outerperipheral surface 3 of the cell structure 1 and is fired, the cellstructure has high strength and the outer wall hardly cracks duringfiring, and thus is suitably usable for a large-sized honeycombstructure. The maximum diameter of the section of the cell structure cutalong a plane perpendicular to the central axis of the cell structuremeans a diameter (maximum diameter) of a section having the maximumdiameter among the sections of the cell structure cut along the planesperpendicular to the central axis of the cell structure (among optionalsections formed along the central axis). In the case of the sectionalshape being circle, the maximum diameter is a diameter of the circle; inthe case of the sectional shape is oval or ellipse, it is a length ofthe major axis; in the case of a polygon, it is a length of the longestdiagonal line; and in the case of indeterminate shape, it is a length(distance between two points) of a portion having the longest distancebetween two points which are taken on the outer periphery of thesectional profile. The honeycomb structure of the present invention isone which is obtained by the above-mentioned method for producing ahoneycomb structure according to the present invention.

In the method for producing a honeycomb structure of this embodiment, itis preferred that kaolin is used as the raw material for the cellstructure 1 and the material for forming outer wall 11 and the averageparticle diameter of the kaolin used as the material for forming outerwall 11 is not less than 1/10 and not more than ½ of the averageparticle diameter of the kaolin used as the raw material for the cellstructure 1. It is generally known that in the case of a cordieritehoneycomb, plate particles such as kaolin and talc in the raw materialare orientated by extrusion molding, and thus the cordierite crystalsafter firing are also orientated to give a low coefficient of thermalexpansion. However, in case a material for forming outer wall (slurryfor the formation of outer wall) is coated, the raw material (material)particles are not orientated and hence it is difficult to obtain a lowcoefficient of thermal expansion such as of a honeycomb (cell structure)which is extrusion molded using plate particles such as kaolin and talcas raw materials after firing. However, when particulate kaolin is usedas a raw material, microcracks are produced and the coefficient ofthermal expansion can be made smaller. Therefore, when kaolin is used asthe raw material for the cell structure 1 and the material for formingouter wall 11 and the average particle diameter of kaolin used as thematerial for forming outer wall 11 is not less than 1/10 and not morethan ½ of the average particle diameter of kaolin used as the rawmaterial for the cell structure 1, the coefficient of thermal expansionof the outer wall 21 can be reduced, the absolute value of thedifference between the coefficients of thermal expansion of the cellstructure 1 and the outer wall 21 can be made smaller, and thus thethermal shock resistance can be improved. If the average particlediameter of kaolin is less than 1/10, increase of firing shrinkagepercentage is caused, and outer wall 21 cracks during firing. Here, theaverage particle diameter is measured by laser diffraction method usingLA-910 manufactured by Horiba Seisakusho Co., Ltd. Furthermore, forimproving the thermal shock resistance, the average particle diameter ofkaolin contained in outer wall 21 of the resulting honeycomb structure20 is preferably not less than 1/10 and not more than ½ of the averageparticle diameter of kaolin contained in the cell structure 1 afterfiring.

In the method for producing a honeycomb structure of this embodiment, asthe cell structure 11, there may be used unfired cell structure and thematerial for forming outer wall 11 may be disposed on the outerperipheral surface 3 thereof, or a previously fired cell structure 1 maybe used and the material for forming outer wall 11 may be disposed onthe outer peripheral surface 3.

In the case of using unfired cell structure 1, the material for formingouter wall 11 is disposed on the outer peripheral surface 3 of theunfired cell structure 1 to produce a cell structure being provided witha material for forming outer wall 10, and this cell structure beingprovided with a material for forming outer wall 10 is fired to producethe honeycomb structure 20. In this case, the step for previously firingthe cell structure 1 can be omitted, and thus the production efficiencycan be improved.

When the cell structure 1 is previously fired, the material for formingouter wall 11 is disposed on the outer peripheral surface 3 of the firedcell structure 1 to produce a cell structure being provided with amaterial for forming outer wall 10, and this cell structure beingprovided with a material for forming outer wall 10 is fired to producethe honeycomb structure 20. In this case, it is preferred to contain aquartz powder in the material for forming outer wall in an amount of1-15% by mass based on the whole material for forming outer wall 11. Inthe case of disposing the material for forming outer wall on thepreviously fired cell structure 1, followed by firing, the cellstructure 1 hardly shrinks in size by the second firing because the cellstructure 1 has been once fired, but when 1-15% by mass of quartz powderis added to the material for forming outer wall, the firing shrinkagepercentage of the material for forming outer wall can be lowered, andhence the absolute value of the difference in firing shrinkagepercentage between the cell structure 11 and the material for formingouter wall can be reduced and thus a honeycomb structure, the outer wallof which hardly cracks during firing, can be obtained. If the quartz isused in more than 15%, the coefficient of thermal expansion of thematerial for forming outer wall increases to cause deterioration ofthermal shock resistance and occurrence of cracks by thermal cycling.

In the method for producing a honeycomb structure of this embodiment, itis preferred that the cell structure being provided with a material forforming outer wall 10 is fired to produce a honeycomb structurecomprising the cell structure 1 and the outer wall 21 provided thereon,and the outer wall 21 is subjected to surface working to make ahoneycomb structure 20 having a given shape. By the surface working, thehoneycomb structure can be finished to one which is higher indimensional accuracy.

In the method for producing a honeycomb structure of this embodiment, itis preferred that the honeycomb structure is cylindrical and thedifference between the maximum diameter and the minimum diameter is 1 mmor less. If the difference is larger than 1 mm, the face pressure at thetime of canning becomes ununiform and the honeycomb structure may breakby the canning. Here, the maximum diameter of the cylindrical honeycombstructure means a diameter of a section having the maximum diameteramong the sections of the honeycomb structure cut along the planesperpendicular to the central axis of the honeycomb structure (amongoptional sections formed along the central axis). Furthermore, theminimum diameter of the cylindrical honeycomb structure means a diameterof a section having the minimum diameter among the sections of thehoneycomb structure cut along the planes perpendicular to the centralaxis of the honeycomb structure (among optional sections formed alongthe central axis).

In the method for producing a honeycomb structure of this embodiment,the outer peripheral surface 3 of the cell structure 1 is formed of thesurface of the outer peripheral wall 5 provided on the outside of thecells positioned at the outermost periphery among a plurality of cells 2as shown in FIG. 1. By forming the outer peripheral wall 5 in this way,the outer peripheral wall 5 can be formed in a given shape with highaccuracy, and hence the shape of the honeycomb structure provided withouter wall 21 can be formed with high accuracy.

According to the method for producing a honeycomb structure of anotherembodiment, in a cell structure 31 produced at an intermediate stage,its outer peripheral surface 33 is formed of the surface of partitionwalls 34 of cells 35 positioned at the outermost periphery among aplurality of cells 32 as shown in FIG. 3. A cell structure beingprovided with a material for forming outer wall (not shown) is producedby disposing the material for forming outer wall on the outer peripheralsurface 33 of the cell structure 31, and is fired to make a honeycombstructure (not shown). In this way, since the surface of the partitionwalls 34 having unevenness serves as the outer peripheral surface 33 ofthe cell structure 31 and the area of contacting (adhering) with theouter wall is great, the adhesion between the outer wall and the outerperipheral surface 33 becomes stronger, resulting in excellent thermalshock resistance, and thus cracks hardly occur due to thermal cycling.The outer peripheral surface 33 of the cell structure 31 may be suchthat at least a part of the cells of the outermost periphery are brokenor at least a part of the cells are ground and the wall surface insidethe cells is externally exposed to form a part of the outer peripheralsurface 33. The method for producing a honeycomb structure of thisanother embodiment is the same as the method of the first embodiment ofthe present invention, except that the shape of the cell structure 31 isdifferent from the shape of the cell structure 1 shown in FIG. 1.

Furthermore, in the method for producing a honeycomb structure accordingto still another embodiment, the outer peripheral surface of the cellstructure may be formed by grinding at least a part of an outerperipheral wall of a cell structure which comprises a plurality of cellsserving as fluid flowing channels and is provided with the outerperipheral wall on the outside of the cells positioned at the outermostperiphery among a plurality of the cells. A material for forming outerwall is disposed on the outer peripheral surface of the cell structure(the portion where the partition walls are exposed to the outerperiphery as a result of grinding) to produce a cell structure beingprovided with a material for forming outer wall, which is fired to makea honeycomb structure. In this way, since the outer peripheral wall ofthe cell structure is ground, the partition walls having unevenness isexposed to outer periphery, and the surface of the partition wallsserves as the outer peripheral surface to increase the area ofcontacting (adhering) with the outer wall. As a result, the outer walland the outer peripheral surface (the portion where the partition wallsare exposed to outer periphery as a result of grinding) are adhered morestrongly at the contacting face of them, resulting in more excellentthermal shock resistance, and thus cracks due to thermal cycling occurmore difficultly. The method for producing a honeycomb structure of thisstill another embodiment is the same as the method of the firstembodiment of the present invention, except that the shape of the cellstructure is different from the shape of the cell structure 1 shown inFIG. 1.

The honeycomb structure of the present invention is one which isobtained by the method for producing the honeycomb structure accordingto the present invention as mentioned above. The honeycomb structurehardly cracks in its outer wall and is high in strength and large insize since it is produced by the method for producing the honeycombstructure according to the present invention.

EXAMPLE

The present invention will be more specifically explained by thefollowing examples, which should not be construed as limiting theinvention in any manner.

The inorganic raw materials shown in Table 1 were weighed in accordancewith the composition of I shown in Table 2. To 100 parts by mass of theinorganic raw materials were added 4 parts by mass of methyl cellulose,0.1 part by mass of potassium laurate and 33 parts by mass of water,followed by mixing and kneading to prepare a clay for molding. This wasformed into a columnar clay by a vacuum kneading machine, from which ahoneycomb (cell structure having outer peripheral wall) was molded by aram type extrusion molding machine. The resulting honeycomb was dried bya dielectric drying machine and completely dried by a hot-air dryingmachine. This honeycomb was cut at both end parts to a given length, andfired by keeping at a maximum temperature of 1425° C. for 4 hours toobtain a cell structure mainly composed of cordierite. The outerperipheral part (outer peripheral wall) was subjected to grinding so asto result in a given outer diameter. Ten cell structures were preparedin this way. Next, raw materials having the compositions A-J shown inTable 2 were weighed, and to 100 parts by mass of the inorganic rawmaterials having each of the compositions were added 0.5 part by mass ofmethyl cellulose, 0.3 part by mass of a dispersant and 44 parts by massof water, followed by mixing and kneading to obtain a slurry for formingouter wall (material for forming outer wall) which had each of thecompositions. The slurry for forming outer wall was coated on the outerperipheral surface of the above cell structure, the outer peripheralpart of which was subjected to grinding, and the coated cell structurewas dried and fired by keeping it at a temperature of 1425° C. for 4hours (Examples 1-6, Examples 9-10 and Comparative Examples 1 and 2).After the firing, the outer wall of the honeycomb structures of Examples1-6 and 9-10 did not crack, while the outer wall of Comparative Examples1 and 2 cracked. The outer diameter of the honeycomb structures was 241mm, the height was 152 mm, the thickness of rib was 150 mm, and the cellpitch was 1.27 mm. A test piece was prepared by drying and solidifyingthe slurry for forming outer wall, and firing shrinkage percentage wasmeasured on the resulting test piece. Further, using the test piece,coefficient of thermal expansion of the outer wall at 40-800° C. wasmeasured. Furthermore, the resulting honeycomb structures were subjectedto thermal cycle test by the following method. The results are shown inTable 3.

(Method of Thermal Cycle Test)

The honeycomb structure obtained by forming the outer wall on the cellstructure and firing it was kept in an electric furnace of 600° C. for 2hours. The honeycomb structure was taken out in a room of 20° C. andkept for 2 hours. This procedure was repeated ten times, and thereafteroccurrence of cracks on the outer wall was visually examined.

The inorganic raw materials shown in Table 1 were mixed in accordancewith the composition of B in Table 2, and two cell structures (firedproducts B) were produced in the same manner as in the production of theabove cell structures, and slurries for forming outer wall were obtainedin accordance with the compositions of E and B in Table 2 by the abovemethod for producing the slurry for forming outer wall. Each of theslurries having the compositions of E and B for forming outer wall wascoated on the outer peripheral surface of the above cell structure, theouter peripheral part of which was subjected to grinding (fired productB), and the coated cell structure was dried and fired by keeping it at atemperature of 1425° C. for 4 hours to obtain honeycomb structures(Examples 7 and 11). After the firing, there were no cracks on the outerwall of the honeycomb structures of Examples 7 and 11. Furthermore, theresulting honeycomb structures were subjected to the above thermal cycletest. The results are shown in Table 3.

The inorganic raw materials shown in Table 1 were mixed in accordancewith the composition of I in Table 2, and a cell structure (greenproduct I) was produced in the same manner as in the production of theabove cell structures, except that firing was not carried out, and aslurry for forming outer wall was obtained in accordance with thecomposition of B in Table 2. The slurry for forming outer wall havingthe composition B was coated on the outer peripheral surface of theabove cell structure, the outer peripheral part of which was subjectedto grinding (green product I), and the coated cell structure was driedand fired by keeping it at a temperature of 1425° C. for 4 hours(Example 8). After the firing, there were no cracks on the outer wall ofthe honeycomb structure of Example 8. Furthermore, the resultinghoneycomb structure was subjected to the above thermal cycle test. Theresults are shown in Table 3.

TABLE 1 Average particle diameter(μm) Talc 11 Calcined talc 12 Kaolin 111 Kaolin 2 5 Kaolin 3 2 Kaolin 4 1.2 Kaolin 5 0.6 Calcined kaolin 2Alumina 5 Aluminum hydroxide 1.5 Quartz 7 Mullite 9

TABLE 2 Calcined Calcined Aluminum Talc talc Kaolin kaolin Aluminahydroxide Quartz Mullite A 20 19 22(2) 21 8 9 1 0 B 20 19 18(2) 17 10 115 0 C 20 20  7(2) 6 16 16 15 0 D 20 20 10(2) 9 11 10 10 10 E 20 19 18(3)17 10 11 5 0 F 20 19 18(4) 17 10 11 5 0 G 20 19 23(2) 22 8 8 0 0 H 20 20 2(2) 1 19 18 20 0 I 20 19 18(1) 17 10 11 5 0 J 20 19 18(5) 17 10 11 5 0Unit: % by mass The numeral value in the parenthesis in the columnkaolin indicates its grade.

TABLE 3 Composition Firing shrinkage of Percentage (%) Coefficient ofmaterial Material thermal expansion for for (×10⁻⁶/° C.) Thermal formingCell forming Cracks Cell Outer cycle test outer structure outer bystructure wall Occurrence Cell structure wall (1) wall (2) − (1) firing(3) (4) (4) − (3) of cracks Example 1 Fired product I A 0.0 0.5 0.5 None0.5 0.8 0.3 None Example 2 Fired product I B 0.0 0.3 0.3 None 0.5 1.10.6 None Example 3 Fired product I C 0.0 0.1 0.1 None 0.5 1.2 0.7 NoneExample 4 Fired product I D 0.0 0.1 0.1 None 0.5 1.0 0.5 None Example 5Fired product I E 0.0 0.4 0.4 None 0.5 0.9 0.4 None Example 6 Firedproduct I F 0.0 0.5 0.5 None 0.5 0.7 0.2 None Example 7 Fired product BE 0.0 0.3 0.3 None 0.3 0.8 0.5 None Example 8 Green product B 0.2 0.30.1 None 0.7 1.1 0.4 None Example 9 Fired product I H 0.0 0.0 0.0 None0.5 1.5 1.0 Occurred Example 10 Fired product I I 0.0 0.2 0.2 None 0.51.3 0.8 Occurred Example 11 Fired product B B 0.0 0.3 0.3 None 0.3 1.10.8 Occurred Comparative Fired product I G 0.0 0.8 0.8 Occurred 0.5 0.80.3 None Example 1 Comparative Fired product I J 0.0 0.6 0.6 Occurred0.5 0.6 0.1 None Example 2 Material for forming outer wall = slurry forformationof outer wall

It can be seen from Table 3 that cracks hardly occurred in thermal cycletest conducted on the honeycomb structures in which the cell structureand the outer wall were combined so that the absolute value of thedifference between the coefficient of thermal expansion of the cellstructure and that of the outer wall was not more than 0.7×10⁻⁶/° C.

INDUSTRIAL APPLICABILITY

As mentioned above, according to the method for producing a honeycombstructure of the present invention, high-strength and large-sizedhoneycomb structures in which the outer wall hardly cracks during firingcan be produced. According to the honeycomb structure of the presentinvention, the outer wall can hardly crack even when the honeycombstructure is high in strength and large in size.

1. A method for producing a honeycomb structure having an outer wall anda cell structure comprising a plurality of cells serving as fluidflowing channels and partitioned by partition walls, the methodcomprising: preparing a material for forming the outer wall using atleast one material selected from the group consisting of talc, calcinedtalc, kaolin, calcined kaolin, alumina, aluminum hydroxide, mullite andsilica, in proportions such that, after firing, the material for formingouter wall contains cordierite as a main component and contains a quartzpowder in an amount of 1-15% by mass based on the whole material forforming outer wall; forming the outer wall from the material bydisposing the material on the outer peripheral surface of the cellstructure, the material configured, such that the absolute value of thedifference between the proportion of shrinkage in the size of a cellstructure after firing to the size of the cell structure before firingand the proportion of shrinkage in the size of the material for formingouter wall after firing to the size of the material for forming outerwall before firing is not more than 0.5%; and firing the cell structurewith the material for the outer wall disposed thereto, to produce thehoneycomb structure, wherein, the cell structure after firing has asection formed by cutting along a plane perpendicular to the centralaxis whose maximum diameter is 150 mm or more, and wherein kaolin isused as a raw material for the cell structure and the material forforming outer wall, wherein an average particle diameter of kaolin usedas the material for forming outer wall is 1/10 or more and ½ or less ofan average particle diameter of kaolin used as the raw material for thecell structure.
 2. A method for producing a honeycomb structureaccording to claim 1, wherein the main component of the cell structureand/or outer wall is a ceramics.
 3. A method for producing a honeycombstructure according to claim 1, wherein the cell structure beingprovided with a material for forming outer wall is produced so that theabsolute value of the difference between the coefficient of thermalexpansion of the cell structure after firing and that of the outer wallafter firing is not more than 0.7×10⁻⁶/° C.
 4. A method for producing ahoneycomb structure according to claim 1, wherein the cell structurebeing provided with a material for forming outer wall is produced sothat the main component of the cell structure after firing becomescordierite.
 5. A method for producing a honeycomb structure according toclaim 1, wherein the cell structure is unfired, wherein the material forforming outer wall is disposed on the outer peripheral surface of theunfired cell structure to produce a cell structure being provided with amaterial for forming outer wall, and the resulting cell structure beingprovided with a material for forming outer wall is fired.
 6. A methodfor producing a honeycomb structure according to claim 1, wherein thecell structure is previously fired, the material for forming outer wallis disposed on the outer peripheral surface of the fired cell structureto produce the cell structure being provided with a material for formingouter wall, and the resulting cell structure being provided with amaterial for forming outer wall is fired.
 7. A method for producing ahoneycomb structure according to claim 1, wherein the material forforming the outer wall is mainly composed of cordierite after firing. 8.A method for producing a honeycomb structure according to claim 1,wherein the absolute value of the difference between the proportion ofshrinkage in the size of the cell structure after firing to the size ofthe cell structure before firing and the proportion of shrinkage in thesize of the material for forming outer wall after firing to the size ofthe material for forming outer wall before firing is not more than 0.3%.9. A method for producing a honeycomb structure according to claim 8,wherein an absolute value of difference between the thermal expansioncoefficient of the cell structure after firing and that of the outerwall after firing is not more than 0.4×10⁻⁶/° C.
 10. A method forproducing a honeycomb structure according to claim 8, wherein the cellstructure has an outer peripheral surface being formed of the surface ofthe outer peripheral wall provided on the outside of the cellspositioned at the outermost periphery among a plurality of the cells.11. A method for producing a honeycomb structure according to claim 8,wherein the cell structure has an outer peripheral surface being formedof the surface of the partition walls of the cells positioned at theoutermost periphery among a plurality of the cells.
 12. A method forproducing a honeycomb structure according to claim 8, wherein the outerperipheral surface of the cell structure is formed by grinding at leasta part of the outer peripheral wall of the cell structure comprising aplurality of cells serving as fluid flowing channels and provided withthe outer peripheral wall on the outside of the cells positioned at theoutermost periphery among a plurality of the cells.
 13. A method forproducing a honeycomb structure according to claim 8, wherein thesurface of the outer wall of the honeycomb structure is subjected to asurface working.
 14. A method for producing a honeycomb structureaccording to claim 8, wherein the honeycomb structure is cylindrical anddifference between the maximum diameter and the minimum diameter of thecylindrical honeycomb structure is not more than 1 mm.
 15. A honeycombstructure comprising: a cell structure provided with an outer wall onthe outer peripheral surface thereof which is produced by firing a cellstructure being provided with a material for forming outer wallincluding: a cell structure having a plurality of cells serving as fluidflowing channels and partitioned by partition walls and a material forforming outer wall which is disposed on the outer peripheral surface ofthe cell structure and forms an outer wall upon firing on the cellstructure, wherein the outer wall includes such a material that theabsolute value of the difference between the proportion of shrinkage inthe size of the cell structure after firing to the size of the cellstructure before firing and the proportion of shrinkage in the size ofthe material for forming outer wall after firing to the size of thematerial for forming outer wall before firing is not more than 0.5%, andthe outer wall contains at least one material selected from the groupconsisting of talc, calcined talc, kaolin, calcined kaolin, alumina,aluminum hydroxide, mullite and silica so that the material for formingouter wall after firing contains cordierite as a main component andcontains a quartz powder in an amount of 1-15% by mass based on thewhole of the material for forming outer wall, and wherein the cellstructure and the outer wall contain kaolin and the average particlediameter of kaolin contained in the material for forming outer wall is1/10 or more and ½ or less of the average particle diameter of kaolincontained in the cell structure.
 16. A honeycomb structure according toclaim 15, wherein the main component of the cell structure and/or outerwall is a ceramics.
 17. A honeycomb structure according to claim 15,wherein the absolute value of the difference between the coefficient ofthermal expansion of the cell structure after firing and that of theouter wall after firing is not more than 0.7×10⁻⁶/° C.
 18. A honeycombstructure according to claim 15, wherein the main component of the cellstructure after firing is cordierite.
 19. A honeycomb structureaccording to claim 15, wherein the cell structure is a fired cellstructure produced by firing a cell structure being provided with amaterial for forming outer wall comprising an unfired cell structure anda material for forming outer wall disposed on the outer peripheralsurface of the unfired cell structure.
 20. A honeycomb structureaccording to claim 15, wherein the cell structure is a fired cellstructure produced by firing a cell structure being provided with amaterial for forming outer wall comprising a cell structure fired inadvance and a material for forming outer wall disposed on the outerperipheral surface of a fired cell structure.
 21. A honeycomb structureaccording to claim 15, wherein the main component of the outer wallafter firing is cordierite.
 22. A honeycomb structure according to claim15, wherein the cell structure being provided with a material forforming outer wall is produced so that the absolute value of thedifference between the proportion of shrinkage in the size of the cellstructure after firing to the size of the cell structure before firingand the proportion of shrinkage in the size of the material for formingouter wall after firing to the size of the material for forming outerwall before firing is not more than 0.3%.
 23. A honeycomb structureaccording to claim 15, wherein the absolute value of the differencebetween the coefficient of thermal expansion of the cell structure afterfiring and that of the outer wall after firing is not more than0.4×10⁻⁶/° C.
 24. A honeycomb structure according to claim 15, whereinthe cell structure has a section formed by cutting the cell structureafter firing along a plane perpendicular to the central axis whosemaximum diameter is not less than 150 mm.
 25. A honeycomb structureaccording to claim 15, wherein the cell structure has an outerperipheral surface being formed of the surface of the outer peripheralwall provided on the outside of the cells positioned at the outermostperiphery among a plurality of the cells.
 26. A honeycomb structureaccording to claim 15, wherein the cell structure has an outerperipheral surface being formed of the surface of the partition walls ofthe cells positioned at the outermost periphery among a plurality of thecells.
 27. A honeycomb structure according to claim 15, wherein theouter peripheral surface of the cell structure is formed by grinding atleast a part of the outer peripheral wall of the cell structurecomprising a plurality of cells serving as fluid flowing channels andprovided with the outer peripheral wall on the outside of the cellspositioned at the outermost periphery among a plurality of the cells.28. A honeycomb structure according to claim 15, wherein the surface ofthe outer wall of the honeycomb structure provided with the outer wallon the outer peripheral surface of the cell structure is subjected to asurface working.
 29. A honeycomb structure according to claim 15,wherein the honeycomb structure is cylindrical and the differencebetween the maximum diameter and the minimum diameter of the cylindricalhoneycomb structure is not more than 1 mm.